According to statistics promulgated by the Energy Information Administration in 1990, there were approximately 35,000,000 residential water heaters operated by electricity out of a total of approximately 94,000,000 residential water heaters. The high cost of electricity relative to fuels such as natural gas is well known. On an operating cost basis, the cost per Btu of delivered water heating for electrical resistance water heaters is two to two and one-half times that: of natural gas-fired water heaters. The concern for reducing energy consumption and conserving natural resources has given rise to a need for cost-effective substitutes for electrical resistance heating of water. Various substitutes for electrical resistance water heating that operate on electricity are known in the water heating industry. These substitutes include solar-assisted water heating systems, desuperheater water heating systems, and heat pump water heaters.
One example of a heat pump water heater is described in U.S. Pat. No. 4,540,874. The heat pump is located external to a conventional water heater having an electrically resistive heating element. A control system is provided for deactivating the heating element and activating the heat pump in response to an initial surge of electric current to the heating element when the contacts of a tank-mounted thermostat are closed, indicating a demand for water heating. Under normal circumstances, the heat pump is operated until the thermostat contacts open, indicating that the demand for water heating has been satisfied. If there is an abnormal condition detected in the heat pump, the control system deactivates the heat pump and activates the electrically resistive heating element until the demand for water heating has been satisfied.
One disadvantage of this type of system is that it utilizes an existing tank-mounted thermostat to control the heat pump. Such a thermostat is designed to switch a relatively high electric current (e.g., greater than 15 amps) and, therefore, may not be able to reliably switch a relatively low electric current (e.g., less than one amp) control circuit. Further, the tank-mounted thermostat may be of unknown specification and wear (i.e., time in operation). Replacement of the tank-mounted thermostat with a thermostat having a different specification (e.g., set points) may be necessitated.
Yet another disadvantage of using a tank-mounted thermostat to control an external heat source such as a heat pump is that as the water in the tank is heated to a certain temperature (e.g., 130.degree. F.), the condensing temperature of the refrigerant in the heat pump must typically be 20.degree. F. higher (e.g., 150.degree. F.) in order to further heat the water. If R22 refrigerant is used, the compressor discharge pressure must reach approximately 400 psi to achieve the 150.degree. F. refrigerant temperature, which exceeds recommended operating parameters for a heat pump using this type of refrigerant. If the tank-mounted thermostat is set at higher than 130.degree. F., the heat pump will not be able to deliver the required water heating because it would be operating well above recommended limits. Although the system described in U.S. Pat. No. 4,540,874 provides for detection of an abnormal condition in the heat pump and switching to the electrically resistive heating element to complete the water heating, it is not desirable to operate the heat pump until the point that an abnormal condition occurs.